ROME — Reporters, worshipers, and tourists were already waiting on the steps of Santa Maria Della Vittoria church Sunday morning when Cardinal Sean P. O’Malley arrived, a half-hour before he was to say Mass.

As he emerged from his car, he seemed to stiffen slightly, as if unnerved by the attention that comes with being seen as a papabile, or contender for pope — an improbable situation for the quiet archbishop of Boston.

Two days before the start of the conclave to elect the next pope, O’Malley has emerged as a popular favorite in some circles, even as many still doubt the princes of the Roman Catholic Church are ready to elect an American. O’Malley led a poll of a half-dozen Vatican experts in the influential daily Corriere della Sera on Saturday, as well as a separate reader poll the paper published online Sunday.

O’Malley’s brown habit, signifying his Capuchin Franciscan order’s simplicity and solidarity with the poor, has helped him stand out. So has his fluency in Spanish and connection with Hispanic communities, lending him an international appeal. And he is recognized as one of the church’s most effective leaders in dealing with clergy sexual abuse.

Franca Giansoldati, a Vatican correspondent for Il Messaggero, another leading Italian paper, who attended Sunday’s Mass and waited afterward for a chance to greet him, called O’Malley “fantastic” and said she hoped he would be the next pope.

O’Malley has repeatedly dismissed the notion that he is a contender, and he continued to do so Sunday at his titular church. (All cardinals are assigned as honorary patrons to a Roman church; O’Malley’s is Santa Maria Della Vittoria.)

Clad in a red cardinal’s cassock for Mass, he silently climbed the steps of the church — a Baroque masterpiece resplendent with elaborate frescoes and marble sculpture, most notably Bernini’s masterwork, The Ecstasy of St. Teresa.

The Rev. Rocco Visca, a member of the Discalced Carmelite friars who run Santa Maria Della Vittoria, lavished praise on the cardinal as he opened the service, calling O’Malley humble and a friend to the friars. He said he hoped that this would be O’Malley’s last visit as a cardinal and that the church would be his first stop after he became pope.

When he began his homily, O’Malley gestured toward Bernini’s masterpiece and quipped that when he took possession of Santa Maria Della Vittoria, he had kidded the friars that he might bring the sculpture back to Boston. They had replied that Napoleon had already tried that, referring to the French military ruler’s raid of Vatican treasure.

“I want to assure you that after the conclave, I will be back as your cardinal, and probably I will try to take the St. Teresa statue back to Boston,” he said with a laugh.

It remains unlikely, although not impossible, that the cardinals will pick an American in the conclave, which begins Tuesday. The voluble, high-octane Cardinal Timothy Dolan of New York is also considered a serious and well-liked contender.

Veteran Vatican analysts see the race as wide open, with a handful of supposed heavyweights vying for votes, including Cardinals Angelo Scola of Italy, Odilo Scherer of Brazil, and Marc Ouellet of Canada.

O’Malley is seen as a potential compromise candidate if the early ballots do not produce a winner. L’Espresso magazine has called him a serious possibility, as have La Stampa and the major Roman daily Il Messaggero. L’Espresso correspondent Aldo Cazzullo called O’ Malley “the pope with the monk’s habit, the man with a subversive energy, the advocate of radical renewal.”

The paper’s Vatican correspondent, however, did not place him in his top three bets.

But handicapping the race is a risky enterprise. Few cardinals disclose their leanings, and although leaks are appearing in the Italian media, they are unsourced and impossible to verify. The cardinals themselves may not know where they stand.

“This is a momentous occasion, when perhaps the will of God isn’t entirely clear to many of us,” Cardinal Francis George of Chicago said Sunday.

The cardinals’ Masses offered a brief explosion of prelates’ voices following last week’s self-imposed silence, and in advance of the absolute secrecy required in the conclave. For papal contenders, it offered a moment vaguely akin to a final barnstorming tour before a presidential election — a last opportunity to shine before the vote. Most cardinals, however, frown on comparisons to secular politics or any semblance of campaigning and say they see the vote as deeply prayerful and spiritual.

Dolan’s Mass at Nostra Signora di Guadalupe, in the middle-class Rome suburb of Monte Mario, was also packed with “giornalisti.” He seemed to be in high spirits, at one point joking with the congregation about the gift baskets with food brought up to him during Mass. “Maybe I’ll take a little package of candies into the conclave,” he said, to big applause. “Because I hear the food there isn’t so great.”

Speaking to journalists after the service, he toggled between English and Italian, telling Italian reporters, “I’m better after two or three grappas.”

O’Malley, apart from his opening joke, seemed more solemn than usual.

He delivered his homily mostly in Italian. Peter Borre, a Boston lawyer who has fought on behalf of some of the parishes O’Malley closed years ago, and whose first language is Italian, was in the congregation; he said it sounded to him like “accented Italian spliced with Spanish words — close enough for Vatican work, I guess.”

O’Malley preached on the Gospel reading of the day: Jesus’s parable of the prodigal son, who squanders his inheritance and returns home ashamed, only to find his father waiting with open arms in forgiveness.

He implicitly linked the story with the exodus of Catholics from the pews in the West, one of the main challenges that will face the next pontiff.

“One can leave the Father’s house, the Church, for many motives: for ignorance, for lack of acceptance, for negative experiences and scandals, or for spiritual mediocrity,” he said, according to a translation provided by the archdiocese. “We find ourselves before a Father who surprises us with his goodness, a Father great in his weakness, who loves us beyond all common sense.”

He spoke briefly in English, about the days ahead: “The Catholic world is united in prayer, filled with confidence that comes from our faith.”

He declined to speak to reporters afterward, but his spokesman, Terrence C. Donilon, said he seemed more relaxed after the worship service.

“He’s in really good spirits,” Donilon said. “I think he’s looking forward to the opportunity to begin the work” of the conclave.

John Noronha, a professor of theology and art and architecture from Illinois who lives in Rome, where he and his wife offer custom tours of the city, said it was fitting that O’Malley celebrated Mass in a church with a monument to St. Teresa of Avila, who helped reform monastic life in the 16th century.

“He was very down to earth, nothing ostentatious, just giving us the simple and profound message of the Gospel,” he said, “and tying it into what we really need to be focusing on during this conclave.”

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